1. Field of the Invention
This invention relates to vacuum interrupters which provide protection in electric power circuits. More particularly, it relates to an arrangement and method for mounting a vapor shield inside the ceramic insulator forming the vacuum envelope of the interrupter.
2. Background Information
Vacuum interrupters typically have a tubular ceramic capped by end plates to form a vacuum envelope. A fixed contact mounted in the vacuum envelope on a first electrode extending through one end cap, and a moveable contact mounted on a moveable electrode axially slideable through the other end plate form a pair of separable contacts which are opened and closed by movement of the moveable electrode by a mechanism located outside of the vacuum envelope. When the separable contacts are opened with current flowing through the vacuum interrupter, a metal-vapor arc is struck between the contact surfaces. This arc continues until the current is interrupted, typically as the ac current goes through a zero crossing. In order to prevent the metal vapor from condensing on the ceramic insulator, a generally cylindrical metal vapor shield is typically provided between the contacts and the ceramic inside the vacuum envelope. One type of vapor shield, the fixed shield, is electrically tied to one electrode, e.g., the fixed electrode, and therefore, can be physically supported by that electrode. The second common type of vapor shield is the floating shield which is electrically isolated from both electrodes. While it is widely known that the floating shield performs better at high voltages than the fixed shield designs, it is more difficult to mount.
One common arrangement for mounting a floating vapor shield requires that the ceramic be formed in two cylindrical parts with a metal mounting ring sandwiched between. The vapor shield is then secured to this mounting ring, typically by discrete flanges brazed to the shield and the mounting ring. While functionally adequate, this arrangement can have several disadvantages. First, attaching the shield in this manner requires two braze joints which have vacuum on one side and air on the other, thereby providing two potential leak paths. Second, the two cylindrical ceramics must be metalized at both ends, which leads to an increased cost. Lastly, the support mechanism exposes a conductor to the air leading to the need for external insulation for high voltage applications.
For these reasons, it is highly desirable to attach a floating shield inside a single ceramic. One approach has been to crimp the shield around a feature such as a ridge molded as part of the ceramic. This is time consuming and adds a fair amount of cost to the ceramic. Various other approaches to attaching the floating shield to a single piece ceramic have required specialized components which add complexity and cost.
There is a need, therefore, for an improved arrangement and method for securing a floating vapor shield in a vacuum interrupter.
There is a general need for such an improved mount and method of mounting which do not require complex specialized parts or techniques.
More particularly, there is also a need for such an improved mount and method which can be used with a single piece ceramic.
This invention satisfies these needs and others by providing a vacuum interrupter with a single piece ceramic tube having a circumferential groove in an inner surface. A pair of end members form with this single piece ceramic tube a vacuum enclosure. A fixed contact is mounted on a fixed electrode extending through one end member. A moveable contact is mounted on a moveable electrode extending through the other end member and axially reciprocal into and out of contact with the fixed contact. A tubular shield is supported inside the ceramic tube and surrounding the fixed and moveable contacts by a shield mount. This shield mount includes a split ring seated in the circumferential groove in the ceramic tube and projecting radially inward from the inner surface of the ceramic tube into the vacuum envelope where a connection connects the shield to the split ring.
The connection between the split ring and the vacuum shield can take several forms. In a preferred form of the invention, this connection includes a flange fixed to the outer surface of the tubular shield and a braze ring fixing the flange to the split ring. Also preferably, the flange forms a gap with the outer surface of the tubular shield and the braze is formed from a braze ring positioned in the gap where it fixes the split ring to the flange in the outer surface of the tubular shield. In one embodiment of the invention, the flange has a radially extending terminal section which seats on the split ring. Also preferably, the flange extends substantially fully around the tubular shield.
In another aspect of the invention, the connection connecting the vapor shield to the split ring includes a circumferential shield groove in an outer surface of the shield, and an additional split ring installed in the groove in the shield and projecting radially outward. A braze connection fixes the additional split ring in the groove in the shield to the split ring seated in the ceramic tube.
In accordance with another aspect of the invention, the connection connecting the split ring seated in the ceramic tube to the shield comprises an additional circumferential groove in the outer surface of the vapor shield in which the split ring seats directly. This connection can be further augmented by including a braze ring to fix the split ring to the shield.
The invention also is directed to a method of securing a floating vapor shield in a vacuum interrupter which includes the steps of forming a circumferential groove in the ceramic tube, installing a split ring in the groove, and fastening the vapor shield to the split ring. The preferred form of fastening is implemented by providing a flange on the outer surface of the shield and fixing the flange to the split ring, preferably by brazing. In a preferred method, the flange is formed with a gap between the flange and the shield and a braze ring is seated in this gap. The shield is then positioned in the ceramic tube with the flange engaging the split ring. Heat is applied, preferably in a vacuum, to melt the braze ring.
In accordance with another aspect of the invention, the shield is fastened to the split ring by forming a shield groove in the outer surface of the tubular shield, installing an additional split ring in this shield groove, and then brazing the additional split ring to the split ring installed in the ceramic tube. Preferably, the brazing is accomplished by placing the braze ring on top of the additional split ring, inserting the shield into the ceramic tube with the additional split ring seated on the additional split ring and applying heat to melt the braze ring.
In still another aspect of the invention, the tubular shield is fastened to the split ring by providing the tubular shield with a shield groove in an outer surface and installing the split ring in this shield groove as well as in the groove in the ceramic tube. Preferably, the split ring is brazed to the tube shield.
The split ring is installed in the ceramic tube by compressing it to reduce its outside diameter to an outer dimension which is less than the inner diameter of the ceramic tube, aligning the compressed split ring with the groove in the ceramic tube and releasing the split ring to radially expand into the groove.